Production of austenitic steel alloys



June 14, 1960 G. F. TISINAI ErAL 2,9

PRODUCTION OF AUSTENITIC STEEL ALLOYS Filed Dec. 29, 1955 PERGENT CARBON INVHVTORS:

James K. Stan/4y BY George f. 77:17:01 00!! H. 80

MFQZZ United States Patent @fiice Patented June 14, 1960 PRODUCTION on AUSTENIT 1c s'rnnr. ALLGYS George F. Tisinai, Highland, Ind, and James K. Stanley and Carl H. Samaus, Chicago, 11]., assignors to Standard Oil Company, Chicago, 11]., a corporation of In= diana Filed Dec. 29, 1955, Ser. No. 556,214

1 Claim. ((31.148-3) Our invention relates to means for the production of stable austenitic stainless steels Without depending upon nickel as an essential alloying element. More particularly, it relates to the production of austenitic stainless steels by an economically attractive melting process.

The chrome alloy stainless steels are normally ierritic or martensitic in structure. Although austenitic ironchromium-nickel and iron-manganese-carbon alloys are well known, so far as we know, it has not been possible to produce completely austenitic steels without the use of nickel or manganese. These alloys have useful, distinctive properties including a high degree of oxida tion and corrosion resistance but their cost and the strategic significance of nickel and manganese have stimulated a great deal of research by investigators, both here and abroad, in an efiort to produce austenitic alloys by other means. i

The iron-chromium-carbon system, for example, has been studied extensively. Ordinarily carbon-free ironchromium alloys containing more than 13% chromium are ferritic at all temperatures. Below 13% chromium the austenitic field exists above 1550 F. Carbon addi- Itions will expand the austenite field up to about 21% chromium at temperatures above 1550" F. With increased chromium content above 21%, the single phase austenite field is eliminated. Krivobok and Grossman, Trans. Am. Soc. Steel Treat, v. 18 (1930), pp. 760- 807, have reported production of an essentially austenitic structure in a steel containing as high as 21% chromium by addition of 0.62% carbon. 0n the other hand, their work with 28% and 33% chromium alloys of variable carbon content (up to 0.70%) resulted in only minor amounts of austenite.

The effect of nitrogen on the structure of iron and chromium alloys also has been extensively investigated. The most significant work appears to be that of Colbeck and Garner, J. Iron and Steel Inst, v. 193 (1939)., pp. 99-435, who have reported the production or" 24% chromium alloys containing as much as 60% austenite and of 27% chromium alloys containing as much as 50% 'austenite. See also 13. G. Bandel, Archiv. t. d. Eisenhuttenwesen, vol. 11, pp. l39144, 1937, wherein the partial austenitization of a number of chromium alloys by addition of nitrogen is reported. A completely austenitic structure, however, has not been realized for high chromium alloys, insofar as we are aware, except for the single non-reproducible example reported by lQivobok and Grossman, supra, without the use of upwards of about 4% nickel.

Recently, however, as described in co-pending application of Tisinai' and Stanley, Serial Number 525,275, filed July 29, 1955, now Patent No. 2,890,995, granted June 16, 1959, essentially austenitic alloys containing upwards of 21% chromium have been produced. The new products followed the discovery that a critical relationship exists between carbon and nitrogen aliecting austenitization, a relationship which prior investigators appear to have overlooked, perhaps because of the unexpectedness of any cooperation between these minor constituents in this respect or perhaps because of the con fusing fact that nearly all steels contain some concentration of carbon as a constituent impurity. The amount of nitrogen that can be retained by the higher chromium alloys is increased tremendously when the alloys contain up to about 1% carbon, and completely austenitic alloys can be produced when the proportions of carbon and nitrogen are adjusted to a critical relationship for a particular chromium content, apparently through some synergistic action. A hyperbolic relationship exists be tween carbon and nitrogen in the chromium alloy steels which appears to be responsible for stabilization of an austenitic structure. The higher the carbon content, the lower is the nitrogen content necessary to make the resulting alloy completely austenitic. Conversely, the lower the carbon content, the higher is the amount of nitrm gen that is required to make the alloy completely ans tenitic. As the amount of chromium in the alloy is increased, larger amounts of carbon and nitrogen appear to be required for a stable austenitic structure. At the higher chromium contents, e.g. say 27 to 33% chromium, the stable austenite field becomes smaller although use-. ful alloys of stable austenitic structure containing coexisting chromium carbide may be formed over a field of broader range:

The new iron-chromium-carbon-nitrogen alloys have valuable corrosion-resistant properties, including oxidation resistance at elevated temperatures at least up to 90.09. F. and also above 2000 F., of a type associated with austenitic nickeland manganese-containing chrome alloys. In other respects, however, the new austenitic alloys appear to be quite distinctive in properties such as tensile and yield strength, work-hardening and the like.

Althoughthe relationship between carbon and nitrogen appears to have some general applicationto chrome al- 10y steels, alloys of less than about 21% chromium content appear to lack the distinctive corrosion resistance and high structural stability of the new alloys. For example, a 21% chromium steel, even though of substantially austenitic structure, is borderline in oxidation resistance at 2000 F. Also, with chromium contents substantially above 33%, impracticably high nitrogen contents are required to avoid restriction of the austenite field to the vanishing point although the two phase field of austenite plus carbide may have general usefulness,

The addition of nitrogen to conventional low-carbon,v

chromium-iron alloys however gives considerable :difficulty in conventional casting. If more nitrogen than about $4 to ,6 of the chromium content is added, gassyingots result during solidification due to the loss of a considerable amount of the nitrogen introduced. we

have found, however, that when the carbon content of the melt is sufiiciently high, e.g. about 0.2% and ad-: vantageously from 0.3 up to about 1%, loss of nitrogen is minimized, or entirely prevented, so that high nitrogen alloys containing up to as much as 1% nitrogen can be cast directly. When the composition of a molten iron chromiurn' alloy is properly controlled, it solidifies in the completely austenitic state rather than in the expected, ferlitic state. Moreover, the high nitrogen is apparently reaustenitizing for short retained in solid solution I porosity, produced by evolving nitrogen, as in the case of ferritic ingots. 'Alloys can be produced in good quality with a' completely austenitic structure at room temperatures. In casting, the retention of sufficient nitrogen in alloys containing less than about 0.3% carbon may be difficult when operating at atmospheric pressure but can be simplified by use of pressure casting techniques, Cast ingots of completely austenitic'structure can be satisfactorily reduced in size by forging.

Thus, by the present invention, essentially completely austenitic, high chromium stainless steel 'alloys can be produced by forming a melccomprising iron, about 21' to'33% chromium, about 0.20 to 1.0% carbon and sufficient nitrogen added in the form of a stable combination with a metal which is fusible in the range of about 2'000'to 2400" F casting the resulting melt and rapidly without giving rise to excessive austenite field in the iron-chromium-carbon-nitrogen sys-' cooling the cast product to a temperature below about 800"F. The process afiords substantial advantages in production, economy and output compared to'the use of nitrogenization orpowder metallurgy' techniques for producing the austenitic alloys. i

The cast alloy is cooled from about 2200 F. to below about 800 to 900 F. rapidly enough, as by quenching, to prevent transformation of the austenitic structure .to the fer itic structure. Although the change in structure from austenitic to ferritic is sluggish, some decornposi tion of austenite may occur on slow cooling as by air i cooling of large sections[ -Quenching or rapid cooling therefore is desirable. A partially decomposed austenite, however, can be converted to an all. austenite structure by reheating" to about. 2000 to 2400- F.," preferably 2200 F., followed by. rapid cooling to; below 800? F.

Also, although the austenitic grain; size maybe quite large in the cast alloys, it can be refined for control of 'me'chanicall properties by decomposing the alloys to ferrite, carbides and nitrides byheating-'to temperatures within the range of about 900 to 1300 F. followed by periods of time at 2200 to 2300" F. 5

The materials used are charged in the usual'way to asuitable melting furnace. A technical grade of iron orcarbon steel, rather than pureiron, is used since .the' customary steel-making impurities appear -to exert. a beneficia'l stabilizing'eifect, The source-of chromium,

advantageously, is a commercial 'ferrochrome although the metal itself can be used. 'The'source of carbon may bethe steel or the ferrochrome, supplemented by the addition of graphite or a metal carbide. Nitrogen advantageously is supplied by use of a high nitrogen ferroc'hrome. For example, nitrogen ferrochromes of from 3% to-2% nitrogen in steps containing 0.1% car- Metal bo'n and about 70% chromium are available. nitrides such as the chromium nitrides, CrN' and particularly Cr N, the iron nitridesFe N, Fe N and Fe N, silicon nitride Si N and the like also have value. Other ferrochromes are available as. sources of nitrogen and carbon, for example, 3 to 6% carbon ferrochromes of less than 0.025% nitrogen and 0.02 to 2.00% carbon ferrochromes of 0.5-0.75% nitrogen.

' In the broader aspects of the invention, completely austenitic alloys can be produced with chromium contents in the range of about 21 to 33% by using selected tern for four. different chromium levels. Eachpanel'represents a constant chromium content, the ordinates are percent nitrogen and the 'abscissas are percent carbon. The diagram applies toalloys cooled rapidly from above 2200" F; All of the alloys above the curved line's-are ferrite free, While the alloys below the curved lines'contain ferrite. Thus; the drawing indicates the'boundaries between the combined .austenite ('y) and the austenite plus carbide +0) fields, above the lines, and the combined austenite plus ferrite ('Y-l-d) and austenite plus ferrite plus carbide (*y++c) fields, below the lines. It

will be noted that the boundaries resemble the hyperbolic form; is. a hyperbolic relation appears to exist between the content of carbon and nitrogen which is responsible for the stable austenitic structure. The hyperbolic boundary shifts upward systematically as-the chromium content is increased so" that greater amounts of carbon and nitrogen are required toobtain the austenitic structure. Thus, from the Isl-dimensional diagram of the drawing it may be seen that a curvedsurface exists which separates the upper austenitie held from austenite containing ferrite. Although alloys in the austenite' plus ferrite field having a predominantly austenitic structure may have engineering usefulness, the presence of more than about 15 to 20% ferrite willresult in the loss of strength and the loss of the non-magnetic nature, and possibly in the loss of general corrosion resistance commonly considered characteristic of the austenitic structure. The presence of small amounts of ferritehowever may lower the susceptibility of 'the austenite toward'intergranular brittleness, may favorthe resistanceof the alloy to corrosionunder stress, and may beuseful to prevent weld cracking in the event the alloys are welded. The inventioniwill be further illustrated by reference to'specific examples of alloys prepared according to the invention. j' a The steels used were charged from either Armed Iron or billet slabs obtained from a 1030" carbonv steel supplemented with granulated electrode carbon as' required for carbon content. The melts were made in a high frequency induction furnace using an unlined magnesia crucible of either 200 pounds or 10 pounds capacity. 7 r V V v The smaller heats wererinitially charged. Furnace V heats larger than 10 pounds were sla'gged after the initial melt-down. .For these heats, both the standard ferrochromium and nitrogen bearing ferrochromium were used, the nitrogen bearing ferrochromium being added after the standard grade.

Deoxidation was made with'either .an 80% Ni20% Mg or MgFeSi (7% Mg42% Si) alloy, or both, de-

proportions of carbon and'nitrogen in. the respective f ranges of 0.1 to 1.0% and 0.2 to 1.0%. The resulting alloys may be pure, but as in conventional alloy production, they normally contain minor or non-detrimental amounts of constituent elements such as sulfur, phosphorus, manganese and silicon.

In the practice of the invention, weprefer to'use a carbon content in the range of about 0.3 to 1.0% more desirably 0.5 to 1.0%. The austenitic, structure then is provided by using sufficient nitrogen in the. range of about 0.3 to 1%, and with higher chromium contents, above about 0.5%. The resulting alloys appear to show pending upon the quality of the alloy intended. Ferroalloy additions were then made as desired. The. melts were poured immediately into either cast iron .or dry sand molds.

Analytical data on an alloy produced from a 10pound heat, without slagging,'and on a 100 pound heat, with slagging, are set'out in Table I below; The alloys were produced as sound castings and, ascast', were feebly magnetia- On reheating to 2200 F., followed byair cooling, the magnetism disappeared completely,.indicating a completely austeniticl structure. The material from the pound ingot was hot swaged and appeared to have excellent but working properties.

TABLE I Compositions (percent) of cast alloys Magnetic Response Cr N Mn P N1 M0 s1 As Cast 2,200 F.Bl1'

Cool

20.81 0. 4s .23 0.83 .010 .014 0.72 0. 01 0.75 Very weak--- None. 21.05 0. 05 .29 0.86 .013 .024 0. 04 0.01 0.87 Weak D0. 21.15 0. a5 0. 3s 0. s0 0. 018 0.017 0.02 0. 01 1.17 Strong Very weak. 20. 00 0.52 .32 1.02 .011 .022. 0. as 0.02 0.82 Mild None.

20. as 0. 33 0. 05 024 .012 0. 04 0. 05 0.94 Do.

23. 9s 0. as .29 0. 17 014 .020 0. 1s 0. 01 0. 01

Analytical data on wrought alloys produced by the We claim:

above procedure are set out in Table II below. A process for the production of essentially austemtlc TABLE II Compositions (percent) of wrought alloys and phases present after quenching from 2200 F.

Si Mn s P 0 N1 Cr N Phases 0. as 0. s2 0. 013 0. 010 0. as o. 24. 5s 0. 392 Austenltlc. 0. s2 0. 86 0. 017 0. 014 0. 33 0.71 27. 51 0. 532 Do. 1. 01 0. 9s 0. 010 0. 012 0. 47 0. 25 20. 55 0. 200 F-AJ 1.15 0. 9s 0 010 0.008 0. 47 0.25 20.50 0.587 Austenitic.

l A stands for austenite and F for ferrite.

The physical properties of the new austenitic ironchromium-carbon nitrogen alloys reveal distinctive characteristics of potential value in a range of industrial applications. Similar to the iron-manganese-carbon Hadfield steels, the chromium alloys show extreme work hardening. This effect is reflected in their resistance both to cutting and to deformation in the tensile test. Even though this austenite shows a hardness value of only about 20 Rockwell C, it is virtually impossible to cut it with ordinary tool steels although the materials can be cut readily with carbide tools. However, when the alloys are decomposed to ferrite, carbides and nitrides, they can be machined readily with ordinary cutting tools. The alloys then can be restored to the austenitic structure by reheating to greater than 2000 F. and cooling.

In the tensile test, very little localized necking occurs even though reduction of area values of 50 percent are attained. The tensile strength of a 27% chromium- 0.5% carbon-0.7% nitrogen alloy is 138,000 p.s.i. The yield strength 90,000 p.s.i. The alloys, of course, are non-magnetic when in the completely austenitic state. In general the new alloys have value in corrosive environments, particularly at the higher chromium contents, where strength and abrasion resistance are required. The new alloys as such may not be well adapted for use at elevated temperatures since they tend to decompose to ferrite, carbides, and nitrides above about 800 to 900 F. The alloys can be beneficially modified in this respect by the introduction of additional alloying elements, for example, manganese, molybdenum, copper, nickel, cobalt, titanium, or aluminum. In slow bend tests, the austenitic alloys show a desirable degree of flexibility for mechanical shaping operations.

35 stainless steel alloys substantially free of ferrite without incorporating austenitizing amounts of metals efiective therefor such as nickel, manganese, and the like which comprises forming a melt consisting essentially of about 21 to 33% chromium, about 0.2 to 1.0% carbon and an amount of nitrogen added in stably combined form in the range of 0.3 to 1.0% sulficient to maintain the relative proportions of nitrogen to carbon for any particular chromium content above the line in the phase diagram of the accompanying drawing, the balance of the melt being iron except for incidental impurities, casting the resulting melt, rapidly cooling the cast product to a temperature below about 800 F., and thereafter refining the structure of the resulting alloy product by reheating to a temperature in the range of about 900 to 1300 F. to decompose the alloy to a mixed phase product, thereupon heating to about 2000" to 2400 F. to reaustenitize the product, and finally rapidly cooling the reaustenitized alloy to below about 800 F.

References Cited in the file of this patent UNITED STATES PATENTS 1,990,589 Franks Feb. 12, 1935 2,454,020 Weitzenkorn Nov. 16, 1948 2,657,130 Jennings Oct. 27, 1953 OTHER REFERENCES Monypenny: Stainless Iron and Steel, second edition, London, Chapman & Hall Ltd., ,1931, page 503.

Colbeck et al.: Journal of the Iron and Steel Institute, vol. 139, No. 1, 1939. Pages 99P-136P. 

